The present invention generally relates to a method for controlling an internal combustion engine, and is in particular related to the control of such an engine during the warm-up period thereof.
Internal combustion engines typically exhibit relatively poor combustion stability during a warm-up period therefor, and particularly following a cold start of the engine whilst it is at a very low temperature. The combustion stability generally improves as the engine warms up towards its normal operating temperatures. In some engines which are controlled by an engine management system under the control of an electronic control unit (ECU), a warm-up period is defined as the initial operation of the engine until it reaches a predetermined engine operating temperature.
The combustion stability within the engine can be indicated by a coefficient of variance (COV) value. This COV value provides an indication of the degree of variation of the gross indicated torque within each cylinder of the engine. The gross indicated torque is directly related to the peak pressures within each cylinder and may graphically be represented by the area beneath a cylinder pressure trace. Variations in the gross indicated torque generally arise as a result of unstable combustion within each cylinder and hence the COV value is essentially a measure of how stable the engine is running. Typically, a decrease in the COV value would indicate an improvement in the combustion stability of the engine.
It is known practice, particularly in four-stroke engines, to try to improve the combustion stability during the engine warm-up period by running the engine using a richer than usual air/fuel mixture and/or by advancing the ignition timing during this period. These operating parameters have generally been controlled manually or automatically as a function of the engine coolant temperature during the warm-up period. However, tests conducted by the Applicant on it""s direct injected engines have shown that there is no direct correlation between the coolant temperature and the degree of combustion stability for certain types of engines. For example, if an engine at start-up having a coolant temperature of say 20 degrees Celsius is compared to the same engine which had previously been started whilst having a lower coolant temperature and which had since been running for a period of time such that the coolant temperature was now at 20 degrees Celsius, the COV value for each situation could well be very different even though the coolant temperature was now the same.
Tests conducted by the Applicant on certain engines reveal that the COV value of the engine typically progressively decreases following cold start-up of the engine during a warm-up period until it reaches an at least substantially constant value. This constant or steady state COV value is generally the same as the COV value of the engine when the engine is running at normal operating temperatures (ie: the engine has effectively warmed up and a satisfactory level of combustion stability has been achieved).
During the warm-up period, both the average cylinder gas temperature (ACGT) within each combustion chamber of the engine and the temperature of the engine coolant progressively increase. The coolant temperature typically rises as a result of energy transfer in the form of heat from the combustion chambers and cylinder walls to the coolant passages of the engine. It has been found that with steady state running conditions after a period of time following start-up, the temperature difference between the ACGT and the coolant temperature becomes at least substantially constant. This may occur even while the combustion and coolant temperatures continue to increase. The point at which this temperature difference first reaches this substantially constant value generally corresponds to the point at which the COV reaches its low steady state value.
Accordingly, it is desired that certain engine operating parameters are modified during the warm-up period such that the ACGT increases so that the temperature difference between the combustion and coolant temperatures under steady state operating conditions attains the constant value referred to above. This would typically lead to the COV value being the same low steady state value as it would under normal running conditions which in turn would effectively result in the achievement of acceptable combustion stability during the warm-up period. This constant COV value would be achievable across any operating conditions.
Further to the above comments, the Applicant has noted that, for a particular engine configuration started from a given coolant temperature, whilst the time to achieve satisfactory combustion stability may differ depending upon engine operating conditions and more generally how the engine is run following start-up, substantially the same level of energy is always put into the engine to attain this satisfactory combustion stability. This energy is placed into the engine by the combustion of fuel within each combustion chamber of the engine during the warm-up period and therefore the amount of fuel delivered to the engine since start-up correlates to the amount of energy delivered to the engine since start-up. That is, for a particular configuration of engine, the point at which the abovementioned temperature difference and COV value reach a constant value also correlates to a certain amount of fuel being delivered to the engine.
It therefore follows that there is a correlation between the amount of fuel supplied to the engine since start-up and the degree of combustion stability of the engine. To reiterate, the total amount of fuel supplied to the engine since start-up (referred to as the xe2x80x9caccumulated fuelxe2x80x9d) required to reach the above noted low steady state COV value is substantially the same regardless of how long it takes to reach that point, provided that the engine at start-up has the same initial coolant temperature. It is therefore not relevant to the attainment of satisfactory stability whether the engine is operated at high speed or remains at idle until that point is reached as long as the same total amount of fuel from start-up is used.
Accordingly, it is possible to base the degree of offset or modification to individual engine operating parameters during the warm-up period on the accumulated fuel since start-up. That is, the offsets can be set on the basis of how much fuel has been delivered to the engine since start-up.
Alternatively, it should be noted that other means for estimating the amount of energy delivered to the engine during the warm-up period may be used. For example, the energy supplied to the engine may be estimated by way of an accumulated value of the load level of each combustion event during the warm-up period.
It is therefore an object of the present invention to operate with a low COV value during a warm-up period for an engine, this being achieved by the provision of operating parameter offsets based on a certain measure of the energy delivered to the engine during the warm-up period.
It is a further object of the present invention to operate with a low COV value during a warm-up period for an engine, this being achieved by the provision of operating parameter offsets based on the amount of fuel delivered to the engine during the warm-up period.
With this in mind, the present invention provides in one aspect a method of controlling an internal combustion engine during a warm-up period thereof including controlling at least one operational parameter of the engine as a function of at least a certain measure of the energy supplied to the engine during the warm-up period. Preferably, the at least one operational parameter of the engine is controlled as a function of at least the certain measure of energy supplied to the engine during the warm-up period of the engine to thereby provide improved combustion stability during said warm-up period.
Conveniently, control of the at least one operational parameter of the engine may be provided on the basis of a certain measure of the energy supplied to the engine during the warm-up period together with other factors related to the engine operation. For example, engine temperature and the certain measure of energy supplied to the engine during the warm-up period may together be used to control the at least one operational parameter of the engine. Further, in more complex models, other factors such as the energy last due to, for example, incomplete combustion of fuel or heat loss, may be taken account of.
Preferably, the measure of the energy supplied to the engine during the warm-up period is based on the amount of fuel delivered to the engine during the warm-up period.
Alternatively, the measure of the energy supplied to the engine during the warm-up period is based on an accumulated value of the load level of each combustion event during the warm-up period.
Conveniently, the coefficient of variance of the gross indicated torque during the warm-up period is maintained at a relatively low value. More preferably, the coefficient of variance of the gross indicated torque during the warm-up period is generally maintained at the same low constant or steady state value that would result from normal running of the engine subsequent to the warm-up period therefor.
Conveniently, control of the at least one operational parameter of the engine as a function of the total amount of fuel to be supplied to the engine during the warm-up period or an accumulated value of the load level of each combustion event during the warm-up period is also dependent upon an engine temperature at starting of the engine. Normally, the engine temperature is given by the coolant temperature thereof. As will be discussed further hereinafter, the initial engine coolant temperature aids in the determination of to what extent the at least one operational parameter is required to be modified during the warm-up period.
Conveniently, in regard to the operational parameter being controlled on the basis of the accumulation of an amount of fuel supplied to the engine, the warm-up period of the engine is that time taken for the predetermined amount of fuel to be supplied to the engine since the starting of the engine. Hence, the length of the warm-up period is dependant on the running conditions of the engine which essentially determine the time taken for the predetermined amount of fuel to be supplied to the engine. In this regard, it is important to note that the control method of the present invention does not necessarily seek to reduce the warm-up period for the engine. Rather, it recognises that a predetermined amount of fuel is required to be supplied to the engine to complete the warm-up period and uses this predetermined amount of fuel to accurately control at least one operational parameter of the engine to provide satisfactory combustion stability during the warm-up period. Further, the predetermined amount of fuel is also used to determine when accurately control of the at least one operational parameter of the engine in this way can cease.
Nevertheless, as compared to prior art warm-up strategies which rely on monitoring coolant temperature to determine when an engine is warm and hence when offsets on various operating parameters can be removed, the method of the present invention may indeed result in a shorter warm-up period. This is mainly due to the fact that the warm-up period is dependent upon the amount of fuel delivered to the engine and that the operating parameter offsets are able to be removed more accurately based on the delivery of this amount of fuel to the engine. Further, it may in fact be the case that the warm-up period is reduced due to the way in which the engine is operated during the warm-up period, even though the same predetermined amount of fuel is delivered to the engine.
Preferably, the at least one operational parameter of the engine is controlled only up to the time at which the predetermined amount of fuel has been supplied to the engine. Thereafter, the at least one operational parameter of the engine is controlled in the known manner under the ensuing engine operating conditions, typically on the basis of normal running maps.
Preferably, the predetermined amount of fuel to be supplied to the engine which defines to length of the warm-up period is determined by measurements and tests conducted on the engine.
Conveniently, the at least one operational parameter of the engine is controlled as a function of the total fuel supplied to the engine since the starting of the engine when the engine temperature is below a predetermined value. The engine temperature is typically given by the coolant temperature of the engine. Alternatively, the engine temperature may be based on the temperature of part of the engine itself, such as the block or the head, or may be based on the temperature of a specific component of the engine such as a head bolt or an inlet valve.
Further to the above, the method may more particularly include:
a) determining the total amount of fuel required to be supplied to the engine to complete the warm-up period,
b) providing a warm-up map for the at least one operational parameter controlling the operation of the engine,
c) selecting a scaling factor for the at least one operational parameter controlling the operation of the engine, the scaling factor being selected as a function of the actual amount of fuel supplied to the engine since the start of the warm-up period, and
d) using the scaling factor to control the transition from the warm-up map to a normal running map for the at least one operational parameter controlling the operation of the engine.
As alluded to hereinbefore, the required total fuel amount to complete warm-up or the xe2x80x9ctotal accumulated fuelxe2x80x9d may be determined as a function of the engine temperature at the start of the warm-up period. Effectively, the engine temperature is used as a reference to the engine condition at the start of the warm-up period. To this end, the required fuel amount may be plotted against engine temperature in a xe2x80x9clook-upxe2x80x9d map provided by an electronic control unit (ECU). As alluded to hereinbefore, the engine temperature may typically be given by the coolant temperature but may alternatively be given by the temperature of, for example, the block, the head, a head bolt or an engine component.
Preferably, the warm-up map may comprise absolute values for the at least one operational parameter. These values are those required to achieve stable combustion at a predetermined start-up temperature which is significantly lower than the normal engine operating temperature. For example, the values in the start-up map may be based on achieving stable combustion at xe2x88x9210xc2x0 C.
Conveniently, the scaling factor is applied to the difference between corresponding values in the warm-up map and the normal running map for certain engine speed and/or loads for the at least one operational parameter. Hence, reduction of the scaling factor by virtue of the increase in the amount of fuel supplied to the engine since start-up controls the transition from the warm-up map to the normal running map for the at least one operational parameter.
Control of the at least one operating parameter of the engine to provide for satisfactory combustion stability during the warm-up period essentially results in an increase in the average cylinder gas temperature ACGT within the or each combustion chamber of the engine and therefore a corresponding increase in the temperature difference between the ACGT and the coolant temperature of the engine. As alluded to hereinbefore, this temperature difference correlates to the coefficient of variance of the gross indicated torque for the engine and hence by achieving a substantially constant temperature difference, a low and substantially constant coefficient of variance can be achieved during warm-up. Importantly, the at least one operational parameter of the engine is controlled according to the method of the present invention immediately preceding cranking of the engine. That is, satisfactory combustion stability is typically achieved immediately the engine is started.
The operational parameters of the engine controlled according to the present invention may include the air supplied to the or each cylinder per engine cycle (APC), and hence the air/fuel ratio, and the ignition timing. Further, in respect of an engine comprising a dual fluid injection system such as that discussed in U.S. Pat. No. 4,934,329, the start of air injection (SOA) which determines the commencement of fuel delivery to the engine may be controlled. Still further, and particularly in regard to a two stroke engine such as those that have been developed by the Applicant, the position of the or each exhaust valve relative to the respective exhaust port of a cylinder may also be controlled. Notwithstanding the above, the control of other engine operating parameters according to the method as described are considered to be within the scope of the present invention.
The scaling factor for each of the above operational parameters may be determined as a function of the total accumulated fuel supplied to the engine. These functions may be mapped within respective look-up maps for each operational parameter. Depending on the engine temperature measured at the start of the warm-up period, the total amount of accumulated fuel required to complete warm-up may vary, typically decreasing with increasing initial engine temperature. Hence, the start point within each look-up map for the determination of the scaling factors may therefore be selected on the basis of the initial engine temperature. That is, the start point which determines the initial scaling factor to be applied to each operating parameter of the engine is based on the amount of fuel required to be delivered to the engine to complete the warm-up period.
The scaling factor for the above noted operating parameters may normally decrease from a maximum value at the start of the warm-up period to a minimum value at the end of the warm-up period. Therefore, at the end of the warm-up period, each operational parameter will have reached a value representative of its typical setting during normal operation of the engine.
A scaling factor may also be provided in respect of the control of the recirculation of exhaust gas, known as xe2x80x9cEGRxe2x80x9d, to the engine combustion chambers. However, because EGR systems typically warm up more slowly than the rest of the engine, control of EGR may need to be based on a longer time frame than the other operational parameters of the engine. Furthermore, the control of EGR may be different to the other operational parameters in that the degree of EGR may always begin at a zero value at the start of the warm-up period and may progressively increase during and beyond the warm-up period of the engine to a required normal operating level. The period of time to reach this normal level may decrease with increasing initial engine temperature.
Whilst the above comments have been based on controlling the at least one operational parameter on the basis of the amount of fuel delivered to the engine during the warm-up period, it should be noted that similar comments apply in regard to controlling the at least one operational parameter on the basis of some other means which effectively correlates to the amount of energy delivered to the engine during the warm-up period.